This invention relates to communication systems and, more particularly, to an improved apparatus and method for receiving and interpreting data signals being telemetered to the surface of the earth in a logging-while-drilling system.
Logging-while-drilling involves the transmission to the earth's surface of downhole measurements taken during drilling, the measurements generally being taken by instruments mounted just behind the drill bit. The prospect of continuously obtaining information during drilling with the entire string in place is clearly attractive. Nonetheless, logging-while-drilling systems have not yet achieved widespread commercial acceptance, largely due to problems associated with transmitting the measured information through the noisy and hostile environment of a borehole. Various schemes have been proposed for achieving transmission of measurement information to the earth's surface. For example, one proposed technique would transmit logging measurements by means of insulated electrical conductors extending through the drill string. This scheme, however, requires adaptation of drill string pipes including provision for electrical connections at the drill pipe couplings. Another proposed scheme employs an acoustic wave which would travel upward through the metal drill string, but the obvious high levels of interfering noise in a drill string are a problem in this technique. Another scheme, which appears particularly promising, utilizes a drilling fluid within the borehole as a transmission medium for acoustic waves modulated with the measurement information. Typically, drilling fluid or "mud" is circulated downward through the drill string and drill bit and upward through the annulus defined by the portion of the borehole surrounding the drill string. This is conventionally done to remove drill cuttings and maintain a desired hydrostatic pressure in the borehole. In the technique referred to, a downhole acoustic transmitter, known as a rotary valve or "mud siren", repeatedly interrupts the flow of the drilling fluid, and this causes an acoustic carrier signal to be generated in the drilling fluid at a frequency which depends upon the rate of interruption. The acoustic carrier is modulated as a function of downhole digital logging data. In a phase shift keying ("PSK") modulating technique, the acoustic carrier is modulated between two (or more) phase states. Various coding schemes are possible using PSK modulation. In a "non-return to zero" coding scheme, a change in phase represents a particular binary state (for example, a logical "1"), whereas the absence of a change of phase represents the other binary state (for example, a logical "0"). The phase changes are achieved mechanically by temporarily modifying the interruption frequency of the mud siren to a higher or lower frequency until a desired phase lag (or lead) is achieved, and then returning the mud siren to its nominal frequency. For example, if the nominal frequency of the mud siren is 12 Hz., a phase change of 180.degree. can be obtained by temporarily lowering the frequency of the mud siren to 8 Hz. for 125 milliseconds (which is one period at 8 Hz. and one and one-half periods at 12 Hz.) and then restoring the mud siren frequency to 12 Hz. It is readily seen that a 180.degree. phase shift could also be achieved by temporarily increasing the mud siren frequency for an appropriate period of time (i.e., to obtain a desired phase lead), and then returning to the nominal frequency.
In conventional PSK communications, the carrier phase is conventionally changed in alternate directions (that is, alternating lead and lag) so that the net change in carrier phase over a long period of time is close to zero. In a logging-while-drilling system wherein an electromechanical device, such as a mud siren, is employed to impart acoustic waves to the drilling fluid, it is preferable to effect all phase changes in the same direction (i.e. either all lags or all leads) which results in the technique for driving the mud siren more efficient and straightforward. (For example, if all phase changes are achieved by momentary decreases in frequency, it is never necessary to increase the frequency above the nominal frequency, and less drive power is needed for the mud siren. Also, the control circuitry can be less complex). The term "unidirectional" PSK modulation means this type of modulation wherein all phase changes are in the same direction.
The modulated acoustic signal is received uphole by one or more transducers which convert the acoustic signal to an electrical signal. It is then necessary to recover the digital information which is contained in the modulation of the received signal. Briefly, this is achieved by first processing the received signals to extract the carrier signal. The reconstructed carrier is then used to synchronously demodulate the modulated electrical signal.
In the type of system described, the carrier is generally extracted using a carrier tracking loop circuit. The carrier tracking loop is a phase-locked loop that includes a voltage controlled oscillator ("VCO") which is responsive to error signals resulting from differences between the phase of the signal derived from the VCO and the phase of the carrier signal. Applicants have discovered, however, that the unidirectional nature of the phase modulation in the type of system described above tends to cause a problem in operation of the phase locked loop. In particular, since phase changes are implemented by momentary variation of frequency, error pulses are generated in the phase-locked loop each time a data transition occurs. Since the PSK modulation is unidirectional (i.e., momentary frequency modification is always to a lower frequency or always to a higher frequency) these error pulses always have the same polarity. These error pulses can tend to cause undesirable frequency deviations in the carrier tracking loop.
It is an object of the present invention to provide an improved carrier tracking loop for use in detection in a phase shift keying transmission system of the type wherein modulation is achieved by temporary unidirectional modification of carrier frequency.